1. Field of the Invention
The present invention relates to a heat-sensitive recording material that is suitably utilized in fields including computer output, printers such as calculators, recorders for medical-purpose measuring instruments, low-speed and high-speed facsimiles, automatic ticket vending machines and handy terminals, and to a production method thereof.
2. Description of the Related Art
Many heat-sensitive recording materials have been proposed that have on a substrate a heat-sensitive color developing layer containing at least a leuco dye and a developer, and utilize color developing reactions between the leuco dye and developer. Advantages of these heat-sensitive recording materials include: (1) absence of necessity to conduct such complicated processing as image development or fixation, enabling short-time recording by means of a relatively simple apparatus; (2) low level of generated noise; and (3) low costs. Thus, they are widely used as recording materials for use in electronic calculators, facsimiles, label printers, recorders, handy terminals, etc., in addition to their application for copying of books and documents.
As the heat-sensitive recording materials, such materials are demanded that are capable of rapid development high-density color, and of imparting rigidity to the color-developed image and background. Recently, such heat-sensitive recording materials have been frequently used in fields where the fidelity of recorded images is important, such as labels. In particular, heat-sensitive recording materials that offer high stability against plasticizers and oils contained in organic polymer materials used in packages are in demand.
To meet these demands, heat-sensitive recording materials where a protective layer is arranged on a heat-sensitive color developing layer are proposed. As binder resin in the protective layer, polyvinyl alcohol, modified polyvinyl alcohol, or a resin where these alcohols and a water resistant additive are combined is used. For example, a combination of diacetone-modified polyvinyl alcohol and a hydrazine compound is proposed (see Japanese Patent Application Laid-Open No. 08-151412). However, the use of this proposed material for a protective layer presents a problem that water-resisting reaction is accelerated when it is applied as a liquid form, and this coating solution for protective layer becomes thicker over time.
Further, another heat-sensitive recording material using diacetone-modified polyvinyl alcohol as a binder resin in the protective layer and containing a hydrazine compound in the heat-sensitive color developing layer is proposed (see JP-A No. 11-314457). However, in this proposal, there is a problem that the water resistance of the protective layer is insufficient; the coating solution for heat-sensitive color developing layer becomes thicker; or color development of the heat-sensitive color developing layer is inhibited by the hydrazine compound.
Further, another heat-sensitive recording material using copolymerized polyvinyl alcohol containing diacetone acrylamide as a single compound; a water-soluble hydrazine compound; and water-soluble amine is proposed (see JP-A No. 10-87936). However, when the proposed material is used as a protective layer, there is a problem that the water-soluble amine adversely affects the heat-sensitive color developing layer, and thereby background color development occurs, pH control becomes difficult, and the liquid thickening is accelerated depending upon the added amount of the water-soluble amine.
As a method for overcoming these problems, for example, it is proposed to combine a reactive carbonyl group-containing polyvinyl alcohol, a hydrazide compound as a crosslinking agent, and a basic filler (see JP-A No. 2002-283717). In this proposal, however, if the protective layer is applied by using a roller blade coater, undulation occurs over the coating surface, with a difference in the deposited amount of protective layer from one position to another. Consequently, barrier properties, such as resistance to a plasticizer or oil, are decreased in regions where the deposited amount on the protective layer is small, and image discoloration occurs in the printed portions. Further, when the surface of the heat-sensitive recording material has been printed with UV ink or flexo alcohol ink, it results in gray scale variations due to undulation over the protective layer.
As a method for preventing the undulation of the protective layer, for example, JP-A No. 06-270538 specifies a relational expression between shear speed and viscosity of the coating solution for protective layer, and states that a coating solution that satisfies this expression causes no coating undulation. However, the physical properties of the coating solution for protective layer, which resulted in the occurrence of coating unevenness, satisfies the expression; thus the proposed expression is unsatisfactory as a condition for coating solution that does not result in coating undulation.
Further, JP-A No. 08-118808 proposes a heat-sensitive recording material where coating uniformity of the protective layer is improved by setting a contact angle on the surface of the heat-sensitive color developing layer at 70° or less. However, the actual contact angle at which coating undulation occurred is 450, satisfying the above-described condition—70° or less. In addition, in this proposal, as a means to reduce the contact angle, a surfactant, filler or a coloring agent with a smaller surface contact angle is contained within the heat-sensitive color developing layer. However, if these additives are added to the heat-sensitive color developing layer, they may adversely affect the quality of the heat-sensitive recording material. For this reason, this method is not sufficient in improving coating unevenness.
Therefore, the current situation is that no heat-sensitive recording material has yet been provided that causes no printing unevenness, excels in such barrier properties as oil resistance and plasticizer resistance and thus offers excellent printability, and no production method for a heat-sensitive recording material has yet been provided that excels in providing a uniform protective layer because of absence of coating undulation upon application of a coating solution for protective layer and thus enables high-speed coating for improved productivity.